Interspinal prosthesis

ABSTRACT

A prosthesis for use in maintaining an interspinal space is disclosed. The prosthesis may be provided as first and second substantially symmetrical halves. The first and second halves may each have a coupling portion and a process portion. The coupling portions allow the separate halves to be separately fit within the interspinal space and then connected to form a single prosthesis, eliminating the need for cutting connecting ligaments to allow insertion of the prosthesis. Various connection and locking arrangements are disclosed to maintain the halves engaged with each other and the interspinal space.

[0001] The invention relates to an interspinal prosthesis of theintroductory portion of claim 1, a counterpart thereto as well as to amulti-part interspinal prosthesis combined therefrom.

[0002] Such prostheses function as spacers for two adjacent vertebrae inthe case of a defective disk, which would otherwise reduce the distancebetween the vertebrae. The stress on the facet joints is also relievedby the enlarged distance.

[0003] The W099/42051 discloses an interspinal prosthesis of this type,which includes a central piece, which is to be introduced into theinterspinal space and from which a pair of ears arises cranially andcaudially to the right and to the left of the central piece, in order tohold the central piece in the space between the spinous processes(processus spinosus) of two adjacent vertebrae after an implantation. Adisadvantage of this known prosthesis is the fact that the latter is inone piece, which makes the implantation more difficult, so that it isnecessary to remove the supraspinal ligament. The removal of thisligament has the disadvantage that the prosthesis is not held securelyin the dorsal direction. For this reason, it is proposed in theWO99/42051 that the prosthesis be drilled through in the interspinalcentral piece, in order to pass a tape through the borehole, with whichtape the prosthesis can be tied to the spinous processes of the adjacentvertebrae. This procedure is very time-consuming and complicated

[0004] The above discussion of the state of the art is given only toexplain the environment of the invention and does not mean that thecited state of the art was also actually published or publicly known atthe time of this application or its priority.

[0005] The invention is to provide a remedy here. It is therefore anobject of the invention to provide an interspinal prosthesis, which canbe implanted while the supraspinal ligament is retained, so that theoperation, as a whole, can be carried out more gently and the implantsecured dorsally without additional means.

[0006] Pursuant to the invention, this objective is accomplished with amulti-part, interspinal prosthesis, the individual parts of which mayhave different mechanical properties.

[0007] The inventive, interspinal prosthesis includes essentially acentral piece with a central axis, which can be introduced into theinterspinal space, and two processes, which arise at the outer ends andextend radially and diametrically to the central axis and can beintroduced into the space between the spinous processes of two adjacentvertebrae. Furthermore, at the inner end of the central piece, avertedfrom the spinous processes, there is an axial depression, whichaccommodates an essentially symmetrical counterpart to the prosthesis.

[0008] In a preferred embodiment, the central piece has coupling means,with which the counterpart can be fixed to the prosthesis. The couplingmeans therefore are constituted so that the positions of the processesat the prosthesis and the positions of the processes at the counterpartare determined relative to one another when the counterpart is fixed inposition. Preferably, the coupling means consist of a slide lock, whichhas a stop, so that, when the counterpart is attached to the prosthesis,the processes assume their desired positions at the prosthesis and thecounterpart. Instead, of with a slide lock, the two parts can also befixed to one another by a screwed or conical connection. In this case,the prosthesis and the counterpart preferably have a twisting safeguard,so that the counterpart can be introduced only in a particular positioninto the axial depression at the prosthesis. A different configurationof the coupling means consist of at least one elastically deformablecam, which, after the prosthesis and counterpart are assembled, can belocked or snapped into position in the latter.

[0009] In a different, preferred embodiment of the inventive prosthesis,the coupling means comprise at least one elastically deformable cam,which, when the prosthesis and counterpart are being assembled, can bedeformed elastically and, after the prosthesis and counterpart areassembled, can be locked in position in a complementary depression.

[0010] A preferred further development consist therein that thecross-sectional planes, orthogonal to the central axis:

[0011] a) have an area of 50 to 300 mm² and preferably of 70 to two 250mm² through the central piece at its narrowest site in the area of theinner end and

[0012] b) an area of 70 to 500 mm² and preferably of 100 to 450 mm²through the processes.

[0013] Furthermore, the prosthesis is produced preferably from anelastic material, so that the central piece can be elastically deformedradially. A sufficient radial, elastic deformability can be achieved byproducing the prosthesis from a plastic or by producing the centralpiece with cogs, which can be deformed radially and elastically.

[0014] The prosthesis may also be produced from an elastomer, siliconeor a polymer from the polycarbonate family. It is, however, alsopossible to produce a prosthesis from a metallic material, if theelasticity of the prosthesis in the region of the central piece, whichcomes to rest in the interspinal space, can be realized by means ofsuitable mechanical devices.

[0015] In a different embodiment of the inventive prosthesis, the latteris constructed hollow, the hollow walls being collapsible and/orexpandable by filling up the hollow spaces. The collapsible hollow wallshave the advantage that, as a result, greater deformation of theprosthesis is made possible than would be permitted by an elasticmaterial.

[0016] In the region of the inner end, the outer surface of the centralpiece can be smooth or roughened. The adhesion of the bone to theprosthesis can be affected by the configuration of the outer surface,being promoted by a rough outer surface and made more difficult or evenprevented by a smooth outer surface. The surface of the implant, whichis in contact with the bone, can also be protected by embedding hydroxyapatite (HA).

[0017] In a preferred embodiment, the inventive counterpart comprises aninner end, an outer end as well as two processes, which arise at theouter end, also extend radially and diametrically and can be inserted inthe space opposite the prosthesis and between the spinous processes oftwo adjacent vertebras. Moreover, a peg, directed toward the inner end,is mounted at the counter part and can be introduced into the depressionat the prosthesis. With that, an exact alignment of the prosthesis andthe counterpart can be attained during the implantation.

[0018] Like the prosthesis, the counterpart can be fitted out withanalogous, respectively complementary coupling means. Once again, thesecoupling means may comprise a slide lock, elastically deformable cams ora screwed or a conical connection. Likewise, an analogous, respectivelycomplementary twisting safeguard is mounted at the counterpart.

[0019] The preferred embodiment of the inventive, interspinal prosthesiswith a counterpart is distinguished owing to the fact, in the assembledstate, it has a plane of symmetry orthogonal to the central axis, theprocesses of the interspinal prosthesis being at a distance of at least2 mm and preferably of at least 3 mm from those of the counterpart, whenviewed parallel to the central axis. The maximum distance of theprocesses of the interspinal prosthesis from those of the counterpart is15 mm and preferably 12 mm.

[0020] The invention and further developments of the invention areexplained in even greater detail in the following by means of partiallydiagrammatical representations of several examples. In the drawings,

[0021]FIG. 1a shows a section through the preferred embodiment of theinventive, interspinal prosthesis with counterpart,

[0022]FIG. 1b shows a side view of the preferred embodiment of theembodiment of the inventive prosthesis with counterpart, shown in FIG.1a,

[0023]FIG. 2 shows a section through a different embodiment of theinventive interspinal prosthesis with a counterpart,

[0024]FIG. 3 shows a section once again through a different embodimentof the inventive interspinal prosthesis with a counterpart,

[0025]FIG. 4 shows a section through a further embodiment of theinventive interspinal prosthesis with a counterpart and

[0026]FIG. 5 shows a view of a further embodiment of the inventiveinterspinal prosthesis with a counterpart.

[0027] In FIG. 1, the interspinal prosthesis 1 with the counterpart 6 isshown in the assembled state. The central piece 2 of the prosthesis 1,with the inner end 7 of the prosthesis 1, adjoins the counterpart 6. Atthe outer end 8 of the prosthesis 1, the two processes 3 are disposedperpendicularly to the central axis 4 and diametrically opposite to oneanother. In the embodiment shown here, the processes 3 are constructedas halves of an ellipsoid body. The also radial and diametricallyopposite to one another processes 3 of the counterpart 6 are disposedsymmetrically to a plane, which is orthogonal to the central axis 4.Three radial cams 17, which are disposed symmetrically when viewed inthe cross-section of the prosthesis 1 parallel to the central axis 4,protrude at the central piece 2 at the inner end 7 of the prosthesis andengage complementary grooves 18 at the counterpart 6, function astwisting safeguard between the prosthesis 1 and the counterpart 6.Coaxially with the central axis 4, the central part 2 includes adepression 5, which penetrates from the inner end 7 into the prosthesis1 up to a depth T. The counterpart 6 has a peg 16, which is constructedto be complementary to the depression 5 and accordingly, during theassembly of the prosthesis 1 and the counterpart 6, can be introducedinto the depression 5. Furthermore, the prosthesis 1 comprises afixing-in-position bolt 19 with a bolt head 26, which can be broughtinto contact with the outer end 8 of the prosthesis 1. Thefixing-in-position bolt can be passed coaxially with the central axis 4through the prosthesis 1 and locked by means of a slide lock 27 in thepeg 16 of the counterpart 6, so that the prosthesis 1 can be lockeddetachably with the counterpart 6. A borehole 20, coaxial with thecentral axis 4, passes through the fixing-in-position bolt 19 and thecounterpart 6, so that the prosthesis 1 and the counterpart 6 can becollapsed radially.

[0028]FIG. 2 shows a further embodiment of the prosthesis with thecounterpart 6 in the assembled state. The depression 5 passes throughthe prosthesis 1 coaxially from the inner end 7 up to the outer end 8.During the assembly of the prosthesis 1 and the counterpart 6, the peg16 at the counterpart 6 is pushed into the through depression until theinner end 7 of the prosthesis 1 comes up against the processes 3 of thecounterpart 6. Moreover, a borehole 20 is drilled through thecounterpart 6 between the outer end 15 and the inner end 14. Thecoupling means 11 are constructed as a screwed connection, the screw 21being passed through the depression 5 at the prosthesis 1 and throughthe borehole 20 at the counterpart 6 from the outer end 8 of theprosthesis 1 up to the outer end 15 of the counterpart 6 and bolted witha nut 22. In addition, the prosthesis 1 is provided with a hollow space12, so that the walls 13 of the hollow space can be collapsed or, byfilling the hollow space 12 with a filling material, expanded.

[0029] The embodiment, shown in FIG. 3, differs from the embodimentsdescribed above in that the peg 16 at the counterpart 6 is passedcompletely through the depression 5 at the prosthesis 1, so that theinner end 14 of the counterpart 6 aligns with the outer end 8 of theprosthesis 1 furthermore, the counterpart 6 has several boreholes 20,which are continuous from the inner end 14 to the outer end 15 and theaxes of which extend parallel to the central axis 4. The cerclage wires23, by means of which the interspinal prosthesis 1 and the counterpart 6are fixed in position, can be passed through these boreholes 21.

[0030] The embodiment, shown in FIG. 4, differs from those shown in FIG.1 owing to the fact that the coupling means 11 comprise a locking bolt28, which can be passed through the borehole 20, which passes throughthe prosthesis 1 and the counterpart 6 coaxially with the central axis4. The locking bolt 28, with its head 29, can be brought into contactwith the outer end 15 of the counter part 6 and has, at its tip,radially and elastically deformable cams 31, which, when the prosthesis1 and the counterpart 6 are assembled, can be locked in an eccentricrelief 30, the diameter of which is larger than the diameter of theborehole 20, so that the prosthesis 1 and the counterpart 6 are heldtogether. For introducing the locking bolt 28 into the borehole 20, thecams 31 can be compressed perpendicularly to the central axis 4 by meansof axially disposed slots 32, so that the locking bolt 28 can be passedthrough the borehole 20, while, in the assembled state, the cams 31spring back elastically and latch into the eccentric relief 30 at theprosthesis 1. A hole is drilled through the locking bolt 28 coaxiallywith the central axis 4, so that a pin 25 can be passed through it, as aresult of which a radial deflection of the cams 31 is prevented

[0031] In FIG. 5, a further embodiment of the inventive prosthesis 1with a counterpart 6 is shown. At the outer end 8 of the prosthesis 1 aswell as at the outer end 15 of the counterpart 6, the processes 3 aremounted once again perpendicularly to the central axis 4 anddiametrically opposite to one another, the processes 3 in thisembodiment having a semicircular cross sectional surface parallel to thecentral axis 4. The depression 5 passes through the prosthesis 1 fromthe inner end 7 to the outer end 8 coaxially with the central axis 4. Inthe depression 5, there is an internal thread 36 with a very largepitch. Adjoining the inner end 14, the counterpart 6 once again has apeg 16, which has an external thread 33 that is complementary to theinternal thread 36, so that the prosthesis 1 and the counterpart 6 canbe fastened detachably to one another by means of this screwedconnection. A first saw tooth-like system 34 is mounted at thecounterpart 6 between the peg 16 and the processes 3 and can be broughtinto engagement with a complementary second tooth system 35 at the innerend 7 of the prosthesis 1 during the assembly of the prosthesis 1 andthe counterpart 6 so that, due to the asymmetric configuration of thesaw tooth systems 34, 35, a safeguard is provided against theunintentional detachment of the prosthesis 1 from the counterpart 6.

1-23 (Cancelled)
 33. A prosthesis comprising: first and second halveseach comprising a process portion and a coupling portion, each couplingportion having an axis and configured for insertion into the interspinalspace between adjacent vertebrae, each process portion configured anddimensioned to prevent advancement into the interspinal space; whereinthe coupling portion of the first half is configured to receive at leasta portion of the coupling portion of the second half, the couplingportions having an unlocked configuration in which the halves areaxially separable from one another and a locked configuration in whichthe halves are axially fixed with respect to each other.
 34. Theprosthesis of claim 33, the coupling portion of the first half furthercomprising a recess, the coupling portion of the second half furthercomprising a projection configured to be received within at least aportion of the recess.
 35. The prosthesis of claim 34, the couplingportions of the first and second halves further comprising a key and acomplementary keyway configured to prevent rotation between the firstand second halves.
 36. The prosthesis of claim 33, further comprising alocking feature for configuring the first and second halves to thelocked configuration.
 37. The prosthesis of claim 36, wherein thelocking feature comprises a bolt having a threaded portion configured tobe threadably engaged by a correspondingly threaded portion of theprojection.
 38. The prosthesis of claim 37, wherein the bolt furthercomprises a head portion configured to axially engage a portion of thefirst half such that rotation of the locking member in a first directiondraws the first and second halves together.
 39. The prosthesis of claim36, wherein the locking feature comprises a bolt and a nut, the bolthaving a head configured to engage an outer surface of the first halfand the nut having a surface configured to engage an outer surface ofthe second half.
 40. The prosthesis of claim 36, wherein the lockingfeature comprises at least one wire configured to pass through a bore inthe recess and projection.
 41. The prosthesis of claim 36, wherein thelocking feature comprises complementary threads on at least a part ofeach coupling portion so that the first and second halves may be screwedtogether
 42. The prosthesis of claim 41, wherein the first and secondhalves further comprise complementary ratchet teeth, which, whenengaged, allow relative rotation between the halves in a first directionand prevent relative rotation between the halves in the oppositedirection.
 43. The prosthesis of claim 33, wherein at least a portion ofat least one of the first and second halves is made of an elastomericmaterial.
 44. The prosthesis of claim 33, wherein at least a portion ofat least one of the first and second halves is made of a metallicmaterial.
 45. The prosthesis of claim 33, wherein at least a portion ofat least one of the first and second halves further comprises a surfacefor enhancing bone growth.
 46. The prosthesis of claim 45, wherein thesurface has a roughened profile.
 47. The prosthesis of claim 45, whereinthe surface comprises a hydroxyapatite coating.
 48. The prosthesis ofclaim 33, the coupling portions configured to substantially preventcompression of the interspinal space when the coupling portions areinserted in the interspinal space.
 49. The prosthesis of claim 48, theprocess portions configured to retain the coupling portions within theinterspinal space when the coupling portions are set in the lockedconfiguration.
 50. The prosthesis of claim 33, wherein the first halfcomprises at least one radially-projecting tab and the second halfcomprises a groove, at least a portion of the tab receivable within thegroove when the coupling portions are engaged to prevent relativerotational movement of the first and second halves.
 51. The prosthesisof claim 33, the recess comprising an internal stop surface configuredto axially engage an end surface of the projection.
 52. The prosthesisof claim 33, wherein the coupling portion of the first half comprises across-sectional dimension of from about 50 square millimeters (mm²) toabout 300 mm².
 53. The prosthesis of claim 33, wherein the processportions of the first and second halves each have a cross sectionaldimension of from about 70 mm² to about 500 mm².
 54. The prosthesis ofclaim 33, wherein the coupling portion of the first half is elastic toallow expansion and/or contraction of the coupling portion.
 55. Theprosthesis of claim 33, wherein the first coupling portion comprisesfirst and second recesses forming an internal shoulder, the secondcoupling portion comprises a prong having a shoulder configured toengage the recess shoulder, the prong further having a compressedposition and an uncompressed position, the prong configured to thecompressed position when in contact with the first recess and configuredto an at least substantially uncompressed position when in contact withthe recess shoulder.
 56. The prosthesis of claim 55, the prong furthercomprising an elastomeric material, wherein engaging the prong with therecess shoulder configures the first and second halves in the lockedconfiguration.
 57. The prosthesis of claim 55, wherein the prongcomprises at least one slot configured to render the prong elasticallycompressible.
 58. The prosthesis of claim 57, wherein engaging the prongwith the recess shoulder configures the first and second halves in thelocked configuration.
 59. The prosthesis of claim 58, further comprisinga pin configured to be received within a bore in the prong to preventthe prong from being configured in the compressed position.
 60. Aninterspinal prosthesis comprising: a first half comprising a couplingportion and a process portion, the coupling portion having a bore andconfigured for insertion into the interspinal space between a pair ofadjacent vertebrae, the process portion configured and to prevent itsadvancement into the interspinal space; a second half comprising acoupling portion and a process portion, the coupling portion configuredto be received within the bore of the coupling portion of the firsthalf, the process portion configured to prevent advancement into theinterspinal space; a locking mechanism for axially locking the first andsecond halves together after at least the coupling portion of the firsthalf has been inserted into the interspinal space; wherein axiallylocking the first and second halves together restricts compression ofthe interspinal space.
 61. The interspinal prosthesis of claim 60, thefirst and second halves comprising an assembled condition and anunassembled condition, the coupling portions of the first and secondhalves insertable into the interspinal space in the unassembledcondition, wherein engaging the coupling portion of the first half withthe coupling portion of the second half configures the halves in theassembled condition.
 62. The interspinal prosthesis of claim 60, whereinthe locking mechanism comprises inner threads on the bore of the firsthalf configured to mate with outer threads on the coupling portion ofthe second half such that the first and second halves can be screwedtogether.
 63. The interspinal prosthesis of claim 62, the lockingmechanism further comprising corresponding engageable ratchet teethformed on the first and second halves to prevent unthreading of thefirst and second halves once the halves have been screwed together. 64.The interspinal prosthesis of claim 60, wherein the coupling portions ofthe first and second halves comprise complementary key and keywaysurfaces configured to prevent rotation of the two portions with respectto each other.
 65. The interspinal prosthesis of claim 64, wherein thelocking mechanism comprises a nut and bolt combination, the shank of thebolt receivable in complementary bores in the coupling portions of thefirst and second halves.
 66. The interspinal prosthesis of claim 60,wherein the locking mechanism comprises a shoulder in the first halfconfigured to receive a compressible prong on the second half.
 67. Theinterspinal prosthesis of claim 66, wherein the locking mechanismfurther comprises a pin configured to be received within a bore of thecompressible prong to render the prong substantially incompressible. 68.The interspinal prosthesis of claim 60, wherein the locking membercomprises at least one wire configured to pass through a bore in therecess and projection.
 69. The interspinal prosthesis of claim 60,wherein at least a portion of at least one of the first and secondhalves is made of an elastomeric material.
 70. The interspinalprosthesis of claim 60, wherein at least a portion of at least one ofthe first and second halves is made of a metallic material.
 71. Theinterspinal prosthesis of claim 60, wherein at least a portion of atleast one of the first and second halves further comprises a surface forenhancing bone ingrowth.
 72. The interspinal prosthesis of claim 71,wherein the surface has a roughened profile.
 73. The interspinalprosthesis of claim 71, wherein the surface comprises a hydroxyapatitecoating.
 74. The prosthesis of claim 60, the coupling portionsconfigured to substantially prevent compression of the interspinal spacewhen the coupling portions are inserted in the interspinal space. 75.The prosthesis of claim 74, the process portions configured to retainthe coupling portions within the interspinal space when the couplingportions are in the locked configuration.
 76. The prosthesis of claim60, wherein the first half comprises at least one radially-projectingtab and the second half comprises a groove, at least a portion of thetab receivable within the groove when the coupling portions are engagedto prevent relative rotational movement of the first and second halves.77. The prosthesis of claim 60, wherein the coupling portion of thefirst half comprises a stop surface configured to axially engage thesecond half.
 78. The prosthesis of claim 77, wherein stop surface isconfigured to separate the process portions of the first and secondhalves by an amount in the range of from about 2 mm to about 15 mm. 79.The prosthesis of claim 60, wherein the coupling portion of the firsthalf comprises a cross-sectional dimension of from about 50 mm² to about300 mm².
 80. The prosthesis of claim 79, wherein the process portions ofthe first and second halves each have a cross sectional dimension offrom about 70 mm² to about 500 mm².
 81. A method of maintaining aninterspinal space first and second vertebral bodies, comprising thesteps of: (a) providing a prosthesis comprising first and second halveseach comprising a process portion and a coupling portion, each processportion configured and dimensioned to prevent advancement into theinterspinal space; the coupling portions engageable with each other toaxially fix the first and second halves together; (b) inserting thecoupling portions into the interspinal space between a targeted pair ofvertebrae; and (c) engaging the coupling portions to axially fix thefirst and second halves.
 82. The method of claim 81, wherein step (b)further comprises inserting each coupling portions into the interspinalspace from an opposite lateral side of the space.
 83. The method ofclaim 82, the coupling portions of the first and second halves furthercomprising complementary threads, wherein step (c) further comprisesscrewing the coupling portions together.
 84. The method of claim 82, theprosthesis further comprising a bolt having a head and a threadedportion, the head configured to axially engage a portion of the firsthalf and the threaded portion configured threadably engage a portion ofthe second half, wherein step (c) further comprises the steps ofengaging the bolt with the first and second halves, and rotating thebolt to draw the first and second halves together.
 85. The method ofclaim 82, the prosthesis further comprising at least one wire configuredto pass through a bore in the first and second halves, step (c) furthercomprising the steps of passing the wire through the bore and engagingthe wire with the spinous process of one of the vertebrae.
 86. Themethod of claim 81, wherein at least a portion of one of the couplingportions has a cross-sectional dimension of from about 50 squaremillimeters (mm²) to about 300 mm².
 87. The method of claim 81, whereinat least a portion of one of the first and second halves is made of anelastomeric material.
 88. The method of claim 81, wherein at least aportion of at least one of the first and second halves is made of ametallic material.
 89. The method of claim 81, wherein at least aportion of at least one of the first and second halves further comprisesa surface for enhancing bone growth.
 90. The method of claim 89, whereinthe surface has a roughened profile.
 91. The method of claim 89, whereinthe surface comprises a hydroxyapatite coating.